This invention relates to micro-display projection systems and, in particular, to micro-display projection systems that use a digital micromirror device (DMD) and a TIR prism.
A. Prior Art Projection Displays Employing DMDs
As known in the art, a digital micromirror device (DMD) comprises a panel which selectively reflects illumination light to produce image light, said panel comprising a plurality of selectively adjustable reflecting elements arranged in a common plane, said elements being adjustable between at least a first position and a second position.
A typical projection display using a DMD (e.g., a DMD from TEXAS INSTRUMENTS) has illumination from a light source (e.g., a high pressure mercury arc lamp), a color wheel for field sequential color, and an illumination path that has an integrator and relay optics. The illumination light from the light source strikes the imager display and is modulated by the micro-mirrors at each pixel. Each flipping mirror can direct the illumination reflected from its surface so that it goes towards the projection lens and screen or off into a reject state where it is blocked from getting to the screen.
There are a number of ways to keep the incident illumination bundle of light separated from the outgoing imaging light, i.e., the light that gets to the screen. The first uses physical separation of the illumination bundle and the imaging bundle. The pupil in the projection lens is then located so as to accept light from pixels that are turned xe2x80x9conxe2x80x9d and reject light from any other direction.
The second method uses a TIR prism to separate the illumination light from the imaging light reflected from the DMD imager. A TIR prism has a face that is close to the critical angle of reflection, i.e., it has a face at which light at some angles undergoes total internal reflection and light at other angles passes through the face. The original disclosure of the use of a TIR prism with a DMD imager was in commonly-assigned U.S. Pat. No. 5,552,922 to Simon Magarill. Other patents in which TIR prisms are used with DMD imagers include: Magarill, U.S. Pat. No. 5,604,624; Peterson et al., U.S. Pat. No. 6,185,047; Poradish et al., U.S. Pat. No. 6,249,387; Fielding et al., U.S. Pat. No. 6,250,763; Okamori et al., U.S. Pat. No. 6,349,006; and Magarill, U.S. Pat. No. 6,461,000.
In what has now become the standard configuration for TIR prisms used with DMD imagers, the illumination light comes in at an angle that xe2x80x9ctotally reflectsxe2x80x9d, while in the imaging path, the light that goes to the screen passes through the TIR surface without any reflections. Again, the pupil in the projection lens is located so as to accept light from pixels that are turned xe2x80x9conxe2x80x9d and reject light, if any, from other directions.
FIGS. 1A and 1B are side and front views, respectively, showing a DMD projection system 9 inside a rear-projection cabinet 11. FIG. 2 is an isometric close-up of the same elements, without the cabinet. These two figures represent the elements that one would find in the cabinet of a rear projection television which uses a DMD and a TIR prism assembly typical of the prior art.
As shown in these figures, prism assembly 13 sits in front of DMD imager 15. Projection lens 17 selects the imaging light and sends it to fold mirrors 19, 21 and then to screen 23. All the parts of a prior art Illumination system can be seen in the figures, namely, a light source 25 (e.g., a high pressure mercury arc lamp), a color wheel 27 with motor 33, an integrating tunnel 29, relay lenses 31, a fold mirror 35, and a further relay lens 37.
B. Operation and Orientation of Illumination Light, Imaging Light, and TIR Surface in the Prior Art
On all DMD imagers produced to date, the square pixel micro-mirrors tilt about their corners. This means that the illumination light needs to come at the device at 45 degrees to the device""s horizontal (see discussion below).
FIG. 3 is a front view of a DMD imager which illustrates how the 45 degree requirement has been achieved in the prior art. In this figure, 39 is a single micro-mirror pixel, 41 shows the mirror flipping axis (axis of motion), 43 illustrates the TIR surface reflection, 45 shows the illumination direction, 47 shows an xe2x80x9cundesirablexe2x80x9d light source orientation (see discussion below), 49 shows the illumination-path fold mirror used in the prior art, and 51 shows the xe2x80x9cdesirablexe2x80x9d light source orientation achieved in the prior art through the use of the illumination-path fold mirror (see discussion below).
To interact properly with the micro-mirrors, the illumination light has to come in perpendicular to flipping axis 41. As a result, illumination direction 45 ends up being at 45xc2x0 to the horizontal axis 53 of the imager 15, i.e., the horizontal axis defined by the imager""s horizontal edges. In a system that uses a TIR prism, the air gap surface is inclined across this same imager axis.
Without fold mirror 49, the illumination axis 45 will go all the way back to the start of the illumination, i.e., to light source 47 in its xe2x80x9cundesirablexe2x80x9d orientation in FIG. 3. For many light sources and, in particular, for light sources which employ an arc burner (e.g., high pressure mercury arc lamps), operation of the source is impaired if the arc burner is not substantially horizontal. It is for this reasons that the orientation of light source 47 in FIG. 3 is considered xe2x80x9cundesirable.xe2x80x9d
As shown in FIG. 3, in the prior art, the need for a substantially horizontal orientation for the light source""s axis was achieved by the use of an illumination-path fold mirror 49. This mirror allowed the light source 51 to have an orientation such that the angle of the source""s axis relative to the horizontal axis 53 of imager 15 was acceptable.
FIG. 4 is a further illustration of the use of TIR prisms in the prior art, in this case, an isometric view of just one of the prisms 13a making up prism assembly 13, with DMD imager 15 being shown below the prism in this figure. In actual use, the DMD imager 15 and the prism assembly will typically be located vertically.
FIG. 4 shows the illumination axis 55 coming into the prism 13a and reflecting down to the device at the TIR surface 57. For illustration, a projection 59 of the illumination axis on the plane of the device is shown as a dashed line in FIG. 4. Note that that the projection is at 45 degrees to the horizontal axis 53 of the imager. FIG. 4 also shows flipping axis 41 which again is at 45 degrees to the horizontal axis 53 of the imager. If the positive direction of the horizontal axis is to the right in FIG. 4, then the flipping axis is at +45xc2x0, while the projection 59 of the illumination axis is at xe2x88x9245xc2x0.
FIGS. 5A, 5B, and 5C illustrate light paths through various TIR prism assemblies used in the prior art, where in each figure a single micro-mirror 67 in its xe2x80x9conxe2x80x9d state (+10 degrees) is shown.
FIG. 5A shows the case where illumination light 61 reflects from TIR surface 57 and imaging light 63 passes through the TIR surface. For a prism composed of acrylic (n=1.493) and for a DMD device with a 10 degree mirror tilt and a F/3.0 light cone, the prism angle is about 35 degrees.
FIG. 5B shows the case where illumination light 61 passes through TIR surface 57 and imaging light 63 reflects from the TIR surface. The mirrors of the imaging device operate in the same manner for this approach as for the approach of FIG. 5A. Thus, the illumination still has to come in at an angle for the tilted xe2x80x9con-statexe2x80x9d mirrors and the imaging (on-state) light still comes straight up off the device (normal to the device plane). To satisfy these conditions, the prisms making up the prism assembly have different configurations for the FIG. 5B approach than for the FIG. 5A approach.
In particular, for the FIG. 5B approach, the prism angle is different from that of the FIG. 5A approach because the imaging light coming straight up needs to be totally reflected at the TIR surface. For example, for an acrylic prism and a DMD device with a 10 degree mirror tilt and a F/3.0 light cone, the TIR prism face for the FIG. 5B approach is at a simple 45 degrees to the device plane, rather than at 35 degrees as in the FIG. 5A approach.
FIG. 5C shows a further approach where the optical paths for both the illumination light and the imaging light undergo total internal reflection. In this case, total internal reflection occurs at both surface 57 and surface 65. Otherwise, this approach is like that of FIG. 5B. In particular, as shown in Peterson et al., U.S. Pat. No. 6,185,047, the approach of FIG. 5C, like the approach of FIG. 5B (and FIG. 5A), has employed a fold mirror in the illumination path.
From the foregoing, it can be seen that in the prior art, DMD projection displays which have employed TIR prism assemblies have included a fold mirror in the illumination path. The present invention addresses this deficiency in the art and provides a projection display which employs: 1) at least one light source, 2) at least one TIR prism assembly, and 3) at least one digital micromirror device, wherein the illumination path between the light source and the prism assembly is unfolded, i.e., the illumination path is free of fold mirrors.
Such an unfolded light path not only simplifies the illumination path but also reduces its length. The reduction in length, in turn, allows smaller and/or fewer relay lenses to be used in the illumination path. This reduces the cost, complexity, and weight of the illumination portion of the system and thus ultimately of the entire projection display.
As discussed in detail below, in accordance with the invention, an unfolded illumination path is achieved for a prism assembly in which illumination light reflects from and image light passes through the assembly""s TIR surface by employing in the assembly a prism having 1) an light input face and 2) a TIR face, both of which have (a) two edges which intersect at an angle greater than 90xc2x0 and (b) two edges which intersect at an angle less than 90xc2x0.
FIG. 13 illustrates such a prism where 69 is the light input face and 71 is the TIR face. The edges of the light input face which meet at an angle greater than 90xc2x0 are edges 73 and 75, while the edges which meet at an angle less than 90xc2x0 are edges 75 and 77. The edges of the TIR face which meet at an angle greater than 90xc2x0 are edges 75 and 81, and the edges which meet at an angle less than 90xc2x0 are edges 75 and 83.
For this embodiment, the TIR fold works on the illumination path which leaves the imaging path free to exit straight off the device and then pass through the TIR surface. This has the advantage of requiring the shortest possible back focal length from the projection lens, which makes the projection lens, which typically has a telecentric or near telecentric pupil, easier to design. It also allows the imager device to be in roughly the same orientation as the screen, which is an advantage for rear projection type displays.
With this configuration, the illumination path in the TIR prism needs to see a compound angle to set up proper illumination. As discussed in detail below, there are more constraints on this type of prism in that it must satisfy both the reflection criteria for the illumination and must remain properly oriented to fully transmit the entire light cone of the imaging path.
Additional features of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.